Several DNA metabolic processes are influenced or controlled by genes whose products function in DNA repair. The RAD52 gene of Saccharomyces cerevisiae plays a central role in many of the events in recombination and repair. It controls the repair of ionizing radiation-induced DNA double-strand breaks, radiation- induced spontaneous mitotic recombination, and recombination during meiosis and is required for mutagenesis by some alkylating agents. Utilizing an antibody raised against a Neurospora crassa deoxyrobonuclease, we had shown that an antigenically-related enzyme could be identified from yeast and that this enzyme is controlled by the RAD52 gene. The protein has been purified and is a single-strand exoendonuclease and a double-strand exonuclease with MW = 72,000. It is not the product of the RAD52 gene. Using a lambda gtll vector expression library that contains genomic yeast DNA and the antibody as a probe, we have identified a segment of DNA that cods for cross-reacting material. A sequence from a yeast genomic library has been identified that hybridizes with this segment. Introduction of this into a Rad+ strain leads to enhanced synthesis of cross-reacting material. The genes appear to be essential based on mutation of the cloned sequence and introduction into the genome.